1. Field of the Invention
The present invention relates to an image sensing apparatus and a method of controlling the same and, more particularly, to sensitivity control of an image sensing apparatus in shooting a moving image.
2. Description of the Related Art
Conventional commercially available image sensing apparatuses such as a digital camera record a still image or moving image sensed by an image sensor such as a CCD or CMOS on a recording medium such as a memory card having memory elements and play back the recorded image.
Many digital cameras have a shooting mode to automatically change the sensitivity in accordance with the brightness of a scene to be taken. This shooting mode enables shooting under appropriate shooting conditions. Especially, an image sensing apparatus such as a digital video camera which shoots and records a moving image automatically controls sensitivity, and the photographer rarely adjusts sensitivity manually in accordance with a change in the brightness of a scene to be taken.
In exposure control of a digital camera, an aperture stop and the like optically controls the quantity of incident light on the image sensor, and a mechanical shutter or electronic shutter temporally controls the quantity of incident light. However, when high-sensitivity setting is necessary because, e.g., the object brightness is low, and it is impossible to obtain a sufficient signal amount by only the optical and temporal exposure control, electrical control is performed by amplifying an electrical signal generated by the photoelectric conversion unit of the image sensor.
A digital camera amplifies an electrical signal by several means. FIG. 7 is a block diagram for explaining an example of the flow of an electrical signal and an amplifier in a conventional image sensing apparatus such as a digital camera.
In an image sensor 201, a plurality of pixels 202 each including a photoelectric conversion unit are arranged. An electrical signal generated in each photoelectric conversion unit passes through a vertical output line and is input to each of column amplifiers 203 which are respectively provided for the columns of the plurality of pixels 202. The gain of the column amplifiers 203 is changeable. The gain often has a discrete set value such as 1×, 2×, 4×, 8×, . . . because of, e.g., the restriction of the layout in the image sensor 201 or the restriction of the number of control signal lines to the image sensor 201.
The electrical signal amplified by the column amplifier 203 is input to an output amplifier 204 via a horizontal shift register. The gain of the output amplifier 204 is also changeable. The gain often has a discrete set value because of, e.g., the restriction of the layout in the image sensor 201, like the column amplifiers 203. However, as compared to the column amplifiers 203 which must be provided as many as the columns, the output amplifiers 204 need only be provided as many as the output terminals, and the degree of freedom of gain selection is high.
The electrical signal amplified by the output amplifier 204 is output from the image sensor 201 and input to an analog signal processing circuit (Analog Front End: AFE) 205. The electrical signal input to the analog signal processing circuit 205 undergoes several analog signal processes by a correlated double sampling (CDS) circuit 206 and the like and is input to an analog signal amplifier (Programmable Gain Amplifier: PGA) 207. The gain of the analog signal amplifier 207 is also variable. However, a more continuous gain can be set than with the column amplifier 203 and output amplifier 204 arranged inside the image sensor 201.
The electrical signal amplified by the analog signal amplifier 207 is converted from an analog signal to a digital signal by an A/D conversion unit 208 and then output from the analog signal processing circuit 205 and input to a digital signal processing circuit (Digital Front End: DFE/video engine) 209.
The digital signal processing circuit 209 performs a digital amplification process as well as various kinds of correction processes and developing processes. The gain of the digital amplification process is also variable, and it is possible to more finely and continuously set the gain than by gain setting of the analog signal amplifier 207.
The electrical signal output from the digital signal processing circuit 209 is stored in a memory and used for display on a display device or recording on a recording medium.
Consider the quality of an obtained image. The final noise amount changes depending on the amplification process that has amplified the signal. More specifically, when the column amplifier 203 has amplified the signal, noise that is amplified by the column amplifier 203 together with the signal is only noise generated in the pixels and the vertical output line before the column amplifier 203. None of the noise components generated in the analog signal processing circuit 205, the digital signal processing circuit 209, and the lines which connect these circuits after the horizontal shift register in the image sensor 201 are amplified.
On the other hand, when the digital signal processing circuit 209 has amplified the signal, noise generated in the path prior to the digital signal processing circuit 209 is amplified by the same gain as the signal. Hence, when the digital signal processing circuit 209 has amplified the signal, the final noise amount is larger than that generated by amplification by the column amplifier 203.
That is, when a signal is amplified by an amplifier of a preceding stage, a higher-quality image with less noise can be obtained.
Examine gain adjustment when a change in the quantity of incident light occurs during moving image shooting. When the digital signal processing circuit 209 amplifies the signal, image characteristics such as the sensitivity, noise in the image, and shading change in accordance with the gain applied to the signal. However, since the gain can be finely and continuously set, the gain adjustment is hardly perceivable.
On the other hand, if the column amplifier 203 amplifies the signal, the shading, sensitivity, and the like may not always be proportional to the gain depending on the performance of the image sensor. Additionally, since the gain setting can be set coarsely and discretely, the change in the image characteristics such as visible noise becomes large, and the user may perceive the gain adjustment. As a result, when the object brightness frequently changes, and the gain adjustment frequently occurs, the image may flicker, resulting in annoyance.
For fine and smooth sensitivity setting according to the quantity of incident light, a digital signal processing circuit 209 capable of more continuous gain setting is needed to amplify a signal.
As described above, sensitivity setting of a moving image has the above-described characteristic features depending on the stage of gain change. For this reason, a process of amplifying a signal in a preceding stage using an amplifier arranged in the image sensor is suitable for a still camera which mainly aims at still image shooting with importance on image quality. On the other hand, a process of continuously amplifying a signal using an amplifier in an analog signal processing IC or video engine is suitable for a video camera which mainly aims at moving image shooting with importance on linkage of frames.
More specifically, a surveillance camera needs performance for enabling recognition of, e.g., a human face even in a low illuminance environment. For this purpose, the camera needs to be able to obtain a high-quality moving image at a high sensitivity and also delicately adjust the sensitivity in accordance with a change in the shooting environment. However, if an amplifier in the analog signal processing IC and a video engine amplifies a signal, noise generated at the preceding stage is also amplified, as described above, degrading the image quality. By contrast, an amplifier provided within an image sensor amplifies a signal, since the settable gain value is discrete, the process causes a moving image flicker, resulting in low image quality.
For example, when a person or object crosses in front of the camera in a bright environment, the quantity of incident light on the image sensing apparatus temporarily abruptly decreases and immediately returns to the initial quantity of light. If sensitivity adjustment according to such an abrupt change in the brightness in a short time is done by an amplifier arranged within the image sensor using a discrete gain, the continuity of the image may be poor, resulting in annoying flicker.